Effects of larvae density and food concentration on Crown-of-Thorns seastar (Acanthaster cf. solaris) development in an automated flow-through system.

Coral-eating Crown-of-Thorns Sea stars (Acanthaster spp.) are major contributors to coral reef loss in the Indo-Pacific region. A release from food limitation of their planktotrophic larvae through enhanced pelagic productivity is one of the main hypothesis explaining population outbreaks ('nutrient limitation hypothesis'). To improve the understanding of these outbreaks we developed an automated flow- through larvae rearing system that maintained food (microalgae) at set levels over the course of four 15d experiments. This resulted in stable food concentrations in experimental tanks. Increased algae concentrations had a significant positive effect on larval development and size at 10 and 15 days post fertilization (dpf). Larvae densities had no effect at 10 dpf. At 15 dpf greater larvae densities were associated with declines in larvae size. Larval development was slowed under higher larvae densities. Thus, the effects of algae concentration and larvae density were additive at 15 dpf, with larvae under low densities at a given algae concentration being further developed than those under higher densities. The development of a flow-through system gives greater insight into the effect of algae and larvae concentrations on Acanthaster development, and the system can be applied to further test the nutrient-limitation hypothesis for present and future outbreaks.